1. Field of the Invention
The invention relates to oscillator circuits, and more particularly to a new circuit structure for reducing the phase noise of an oscillator circuit using an LC tank.
2. Description of the Prior Art
High frequency voltage controlled oscillators (VCO) are widely used in communication systems. A key attribute for VCO circuits is the ability to have low phase noise and high signal to noise ratio (SNR).
In U.S. Pat. No. 6,064,277, an oscillator circuit is described. Please refer to FIG. 1. FIG. 1 is a schematic diagram of a prior art oscillator circuit 10. The prior art oscillator circuit 10 uses a drive circuit 12 to drive an LC tank. The LC tank comprises an inductor L and a capacitor C, and provides sinusoidal oscillation for the oscillator circuit 10. The drive circuit 12 has two passive components Z1 and Z2. Z1 and Z2 may be resistors or inductors. Z1 and Z2 are connected to the bases of biasing transistors Q1 and Q2 respectively. The drive circuit 12 is fed with a bias voltage VBIAS, which serves as an anchor voltage for the biasing transistors Q1 and Q2 through Z1 and Z2. Typically, Z1 and Z2 have large resistance values in order to reduce the loading effect on the bias voltage source.
However, the passive components Z1 and Z2 cannot completely block noise introduced by VBIAS. Moreover, the large resistance of the passive components Z1 and Z2 will also contribute thermal noise into the oscillator and thus introduce phase noise. As a result, a signal to noise ratio (SNR) of the prior art oscillator circuit 10 is limited.
It is therefore a primary objective of the claimed invention to provide a voltage controlled oscillator to solve the above-mentioned problem.
According to the claimed invention, a voltage controlled oscillator (VCO) has an LC tank for generating a sinusoidal wave and a positive feedback circuit electrically connected to the LC tank for amplifying the sinusoidal wave. In addition, the VCO includes a noise reducing circuit comprising a T-shaped resistor configuration connected to the positive feedback circuit. The use of the T-shaped resistor configuration allows the VCO to reduce phase noise generated by the LC tank, and provide a high SNR. Alternatively, the noise reducing circuit can use a delta-shaped resistor configuration instead of the T-shaped resistor configuration. The delta-shaped resistor configuration includes a first resistor connected to the positive feedback circuit for reducing phase noise generated by the LC tank and two second resistors connected to the first resistor for receiving a DC bias voltage.
Another objective of the present invention is to provide a common-mode compensation circuit that can eliminate the common-mode noise in the common node of the LC tank. Using a pair of junction diodes to sense the common-mode signal from the differential output, such compensation circuit can prevent two transistors connected to the LC tank from deep saturation. When the transistors operate in deep saturation mode, a negative compensation is provided to reduce the phase noise.
It is an advantage of the claimed invention that the VCO includes a noise reducing circuit with a T-shaped resistor configuration or a delta-shaped resistor configuration and a compensation circuit to reduce phase noise and provide superior SNR.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.